When you punch yourself, then do it not only on the lower belly (hight of the navel), but also up high into the stomach / solar plexus area just below the sternum. It works best when you fully relax your abs and breathe in.
If someone else would punch you that way, the very first punch would make you clutch your belly, double up, drop to the floor, assume fetal position, beautifully grimace in pain and utter highly erotic moans while trying to get some air into your lungs.

Punch me hard the way you describe, make me clutch my big belly and drop to the floor!

I'll happily give you those 5 shots :)

My gut is waiting! Got the guts to take me on?!

Oh yes I do

Right in my big belly!